1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use in, for example, high-frequency circuits in radio communication devices.
2. Related Art of the Invention
The electromechanical components using a surface acoustic wave have received attention in the tide toward higher density of hardware due to the following fact, that is, the acoustic velocity of the wave is several km/s, and the energy of the wave has a property of concentrating on the surface of a propagation medium. Also, with the advance of thin film forming technology and surface processing technology which have enabled the development of an interdigital transducer (below, referred to as IDT) electrode, and the transformation and expansion thereof, they have been applied to practical use for radar delay lines and band-pass filters for television receivers. At present, they have come in wide use as filters of RF and IF stages in transmitter-receiver circuits of radio communication devices. In recent years, there has been under way the increasing development of portable telephones, digital cordless telephones, and the like with the digitization of mobile communication devices. This results in an expanded occupied bandwidth per channel. Especially, IF filters for use in a CDMA (code division multiplex) which has attracted attention in recent years have been required to have characteristics excellent in broad-band and extremely flat group delay time deviation and selectivity for differentiating between adjacent-channel signals and desired signals.
Conventionally, as surface acoustic wave filters suitable for an IF stage, resonator type surface acoustic wave filters and transversal type surface acoustic wave filters are well known. The resonator type surface acoustic wave filter has narrow-band and steep cut-off characteristics, and also small insertion loss and device size, while being inferior in group delay time characteristic. On the other hand, the transversal type surface acoustic wave filter has large insertion loss and device size, while having a broad-band and flat group delay time deviation characteristic. From the foregoing features, the transversal type surf ace acoustic wave filter is suitable for use as IF filter in CDMA(code division multiple access).
Below, a description will now be given to a conventional transversal type surface acoustic wave filter.
FIG. 15 is a block diagram showing a transversal type surface acoustic wave filter in accordance with prior art. Referring now to FIG. 15, the reference numeral 151 denotes a single crystal piezoelectric substrate. On the piezoelectric substrate 151, is formed an electrode pattern, which enables the excitation of a surface acoustic wave. The reference numeral 152 represents an input IDT electrode pair, from which an output IDT electrode pair 153 is formed at a prescribed distance, thus forming a transversal type surface acoustic wave filter.
With the surface acoustic wave filter configured as described above, various weights are assigned to the input and output IDT electrode pairs 152 and 153, alternatively, any one of, or both of the input and output IDT electrode pairs 152 and 153 are set to be a unidirectional IDT electrode pair, or the like. This determines the frequency characteristics of the filter, thus implementing the filter characteristic with a broad bandwidth and being flat within the passband.
Incidentally, a reduction in size and weight of portable remote terminals has proceeded in recent years, with which miniaturization has been also required of the surface acoustic wave filter of the IF stage. Further, the balanced input and output of ICs in stages around that of the IF filter has advanced, and the balanced input and output type has been also required of the IF filter strongly. Excellent attenuation amount outside the passband has been also required for differentiating between adjacent-channel signals and desired signals.
However, in the case where steep attenuation characteristics in the vicinity of the passband is obtained with the above-described transversal type, sufficient weighting is required of the input and output IDTs. This entails a problem that each length of the input and output IDTs is increased, which leads to a difficulty in miniaturization thereof. Further, weighting results in a difference in configuration between the upper electrode and the lower electrode of the IDT electrode pair 152, which creates a problem of bad balancing.